Rock drilling apparatuses may be used in a number of areas of application. For example, rock drilling apparatuses may be used in tunnelling, underground mining, rock reinforcement, raise boring, and for drilling of blast holes, grout holes and holes for installing rock bolts, etc.
A drill tool such as, for example, a drill bit is often used during drilling, the drill bit being connected to a drilling machine, in general by means of a drill string. The drilling can be accomplished in various ways, e.g. as rotational drilling where the drill tool is pushed towards the rock at high pressure and then crushes the rock by means of rotation force and applied pressure.
Percussive drilling machines can also be used, where, for example, a piston strikes the drill string to transfer percussive pulses to the drill tool via the drill string and then further on to the rock. Percussive drilling is often combined with a rotation of the drill string in order to obtain a drilling where the buttons of the drill bit strikes fresh rock at each stroke, thereby increasing the efficiency of the drilling.
During drilling the drill tool can be pressed against the rock by means of a feed force to ensure that as much impact energy as possible from the hammer piston is transmitted to the rock.
The above drilling principles have in common that the rock is crushed during drilling, whereby drilling remnants, so called drill cuttings, are formed and which must be evacuated from the drill hole in order to perform the drilling in an efficient manner.
This is in general performed with the aid of a flushing medium, such as, for example, compressed air, flushing air, which is led through a channel in the drill string for release through flushing air holes in the drill bit to thereafter bring drill cuttings on the way up through the hole.
During rock drilling, such as, but not limited to, top hammer drilling, there is a risk that the flushing air holes in the drill bit gets clogged by drilling remnants during drilling, and thereby stops the flushing air from flushing away the drilling remnants. If the flushing air is stopped from flushing the hole clean from drilling remnants, the drilling remnants will start to build up on the drill bit, which leads to deteriorated drilling and the drill bit in a worst case getting completely stuck.
Consequently, systems for detecting and stopping such situations from arising are required, e.g. by generating a warning signal if the flushing air flow falls below a too low level, whereby suitable actions can be taken.
Today, a so called venturi tube, which is arranged between compressor and drill string, is, in general, used at drilling rigs where a flushing medium consisting of compressed air is used. A pressure switch is measuring the differential pressure over the venturi tube, where the pressure difference over the tube increases with an increasing flow through the tube. The pressure switch is set such that a signal is generated when the pressure difference over the venturi tube, and thereby also the flushing air flow, is lower than a set level.
This solution, however, has several disadvantages. Apart from the solution being relatively expensive, sensitive and difficult to set in a correct manner, the pressure switch consists of an analogue sensor that cannot be controlled, e.g. via software. Due to difficulties in setting the pressure switch, which in general is carried out manually by means of e.g. adjuster screws, it is also not possible to adapt the pressure level difference at which the pressure switch will generate a signal to different operating points, which means that the pressure switch can function better at certain conditions occurring during rock drilling as compared to other situations with other prevailing conditions.
Consequently, there exists a need for an improved method for determining variations of the flushing medium flow during rock drilling.